This invention relates to optical amplifiers and is concerned more particularly, but not exclusively, with multiple stage erbium doped fibre amplifiers (EDFAs).
The performance of optical amplifiers can be characterised by two main performance parameters, namely the noise figure (NF) and the Gain Ripple (GR). In an inverted erbium doped fibre amplifier (EDFA) the signal is amplified by stimulated emission with a noise component generated as a result of spontaneous emission. This spontaneous emission can be further amplified by the amplifier, resulting in amplified spontaneous emission (ASE). ASE is thus generated over the entire gain bandwidth region of the amplifier. In conventional EDFAs the gain and or power of the amplifier is controlled using total power monitoring schemes that cannot not differentiate between signal and ASE components. The noise figure and gain ripple are thus impacted by the effectiveness of the control scheme and its account of ASE.
U.S. Pat. No. 6,606,191 discloses a method for controlling the performance of an optical amplifier in order to compensate for ASE in a gain control mode. In this method values are obtained for the input and output power electronic circuitry DC offset, and set point values are selected as follows: power set point=—0.0, gain set point=_desired gain. A digital value representative of the signal input value is then obtained and the input electronic circuitry DC offset subtracted to produce an adjusted input power value, and a digital value representative of the signal output value is obtained and the output power electronic circuitry DC offset subtracted to produce an adjusted output power value. The ASE content of the adjusted input power value is obtained either utilising a look-up table or a formula, and the ASE content is subtracted from the adjusted input power value to produce an ASE adjusted output power value. The gain set point is multiplied by the ASE adjusted input power value and subtracted from the ASE adjusted output power value, or alternatively the ASE adjusted output power value is divided by the gain set point and the adjusted input power value is subtracted from this to obtain the gain control error signal. The gain control error signal is then subtracted from any number to produce a controller input signal, and the controller input signal is transformed into digital representation of at least one pump control signal by a control algorithm. The digital representation is transformed into at least one corresponding analogue pump control signal that is applied to at least one pump by way of driver circuitry.
Although the input power is adjusted for its overall ASE content in such an arrangement, there is no mention of making adjustments for the ASE content arising in the gain stage doing the work. Accordingly the ASE is amplified along with the signal and without taking into account ASE from the current stage. In a multistage amplifier the ASE compensation of any stage after the first gain stage must account for both the ASE arising from the preceding stages as well as the ASE arising from that gain stage. It follows that the amplifier cannot be switched to the power control mode using ASE factors determined from calibration in the gain control mode, so that, in this case, specific calibration of ASE factors for the power control mode would be required.
Furthermore U.S. Pat. No. 6,525,873 discloses an optical fibre amplifier comprising at least one optical pump source coupled to an optical gain medium, an input optical signal tap coupled to a first optical detector, an output optical signal tap coupled to a second optical detector, and an electronic controller receiving inputs from the detectors to control the optical power provided by the pump source. The controller includes a programmed unit and a user interface allowing the user to choose between available control functions including ASE noise compensation. Although this reference does not specify how ASE calibration is effected, it seems likely that such ASE compensation requires separate calibration in the gain control mode and in the output power control mode.
It is an object of the invention to provide an optical amplifier which enables ASE compensation to be effected in a particularly straightforward manner.